1. Field of the Invention
The present invention relates to scanning systems and more particularly to such systems capable of providing a linear scan.
2. Description of the Prior Art
In many scanning and recording applications, it is required that the scan record spot move in a straight line with essentially constant velocity. Such a requirement is particularly important when the rigidity or other characteristics of either the scanned material or the recording medium preclude any bending. The conventional prior art approach to such straight line scanning is one utilized in nearly all scanning microdensitometers. In such prior art devices, a small spot of light is formed by a microscope type of optical system and is caused to traverse the material being scanned either by movement of the spot forming optical assembly or by movement of the material being scanned with the optical assembly. However, this prior art approach is limited to applications which can be satisfied with a relatively low scan velocity, such as a few inches per second, because of inertial effects which are present at the beginning and end of the scan. In an attempt to eliminate such inertial effects, prior art straight line scanners and recorders have utilized flying spot cathode ray tubes in which the moving spot on the cathode ray tube phosphor is reimaged optically on the surface being scanned. However, such flying spot scanners are limited by the finite size of the spot on the phosphor and the usable cathode ray tube diameter. In an effort to extend this limit, multiple cathode ray tube configurations have been utilized resulting in a complex system. In addition, in such prior art flying spot scanning systems, the radiance of the flying spot is relatively low requiring, in recording applications for example, high sensitivity photographic material. Because of these limitations, most high speed scanning/recording systems which require a small spot of high brightness conventionally utilize a rotating or oscillating mirror surface to cause the deflection of the spot forming beam which is provided either by an incoherent source, such as a tungsten lamp, or a coherent source, such as a laser, in which increased image brightness may be obtained.
Rotating multifaceted reflectors are utilized in prior art high speed applications to normally avoid the time required for an oscillating reflector to stop, start and possibly retrace between scans, these prior art rotating reflectors either being prismatic in which all reflecting faces are parallel to the axis of rotation, or pyramidal in which the reflecting faces are at an angle to the axis of rotation. In either of these cases, the number of faces is normally determined by the geometrical requirements of the scan/record system. The reflecting face in such prior art systems may either be overfilled or underfilled by the imaging radiation. In the instance when the reflecting face is overfilled, the boundary of the face is the aperture stop of the system, such overfilling usually resulting in less efficient use of available light particularly when laser sources are utilized, overfilling being generally utilized, however, when duty cycles approaching 100% are required. In the instance when the reflecting face is underfilled, the aperture stop of the system is normally determined by some other external obstruction. Furthermore, in such prior art scanning/recording systems, the reflecting face is normally located in either collimated or focussed space. If the reflection takes place in collimated space, the subsequent spot forming optics operate over the full scan angle; however, if the beam is focused before deflection, the prior optics operate in an on-axis condition enabling a simplier optical design than required for operation in collimated space. However, in such prior art prefocusing systems, the deflection occurring after focusing results in the locus of the scanning spot being inherently curved as opposed to the desired straight line or linear scan.
In an attempt to overcome this problem in prior art systems employing prefocusing, the scanned surface and/or recording medium must be cured on to a cylindrical type of surface, such as the system disclosed in U.S. Pat. No. 2,853,918, and is not satisfactory for providing a straight line or a linear scan when the scan surface or record medium is in a flat plane. In addition, in attempting to compensate for the curved scan, such as in a laser scanning system, to provide a flat plane scan, prior art systems have employed parabolic compensating mirrors in which corrections for distortions due to vertical non-linearity are provided, such as the type of system disclosed in U.S. Pat. No. 3,441,949. However, such prior art systems do not provide compensation for horizontal nonlinearity and distortions still exist, therefore, in curvature of scan in the horizontal plane. Another elaborate prior art compensating system which has not proved satisfactory is disclosed in U.S. Pat. No. 3,469,030 in which field flattening lenses are utilized in conjunction with a beam splitter to compensate for optical displacement inherently caused by the beam splitter. These disadvantages of the prior art are overcome by the present invention.